In the outward appearance, the earth is a nearly spherical ball with a radius of 6350 kilometres. Internally, the earth consists of three major layers. These are :
We shall now discuss the nature of these three layers of earth in detail. Let us start with the crust.
The Crust of Earth :
The outermost layer of earth is called crust (see Figure 16 on next page). The crust is made up of light rocks rich in silica and aluminium and having a low density of about 3 g/cm3. The crust of earth is like the skin around an orange or an apple. The thickness of earth’s crust is not uniform throughout, it changes from place to place. The thickness of earth’s crust is maximum under the continents and minimum under the oceans (seas).
For example, under continents (land masses), the thickness of earth’s crust is from about 35 kilometres to 60 kilometres. The thickness of earth’s crust under the oceans is only about 10 kilometres. The three-quarters (3/4) of the earth’s crust is covered with water. Over the earth’s crust is a layer of atmosphere. Right from the day of formation of earth, its crust has been constantly broken and then reorganized to form new continents and oceans. It is this thin and solidified crust of earth which supports all life. The earth’s crust is of lighter density as compared to the interior layers.
The Mantle of Earth :
The central region between the crust of earth and core of earth is called mantle (Figure 16). The mantle of earth extends to a depth of about 2900 kilometres below the crust. The mantle of earth consists of rocks which are made up largely of iron silicate and magnesium silicate. The pressure inside the mantle increases with depth. Even under high pressure in the mantle, the rocks are usually in the solid state. Under extreme pressures and temperatures, however, some of the rocks near the bottom of the mantle tend to flow like coal-tar. The density of mantle increases slowly as we go downwards. In the beginning of mantle, the density is about 4 g/cm3. It goes on increasing until it reaches a value of about 6 g/cm3 at the bottom of mantle. Here, the mantle ends and a sudden increase in density occurs. It is here that the core of earth begins.
The Core of Earth :
The innermost part of earth is called its core (see Figure 16). The core of earth has a radius of about 3400 kilometres and it is extremely hot. The core of earth is made up of mainly iron (and some nickel). The core of earth actually consists of two parts : the inner core and outer core. The inner core is a solid sphere of highly compressed iron whereas the outer core is of molten iron.
Thus, the core of earth consists of an inner solid sphere of highly compressed iron, surrounded by an outer shell of molten iron. The temperature at the centre of the earth is about 4000°C and the pressure is extremely high, being about 3.7 million atmospheres (3.7 x 106 atm). It is because of this extremely high pressure that the iron in inner core can remain in the solid state despite the very high temperature of 4000°C (which is much above the melting point of iron, 1539°C). Since the pressure is comparatively lower in the outer core, therefore, the iron metal remains in the molten form (liquid form) in the outer part of the core of earth. Thus, about 1600 kilometres of the outer core behaves like a liquid. Since the core of earth is very hot but the surface of earth is colder, so heat flows outwards from the hot interior of the earth to the surface of earth.
The flow of heat through the outer core of molten iron causes convection currents which produce a magnetic field around the earth. The outward flow of heat energy from the hot interior of the earth to the cooler surface, makes the earth behave as an engine, which transfers energy from one place to the other. The inner core of the earth is very dense, its density being about 18 g/cm3. The density in outer core varies from about 10 g/cm3 just below the mantle to about 16 g/cm3 just before the inner core starts. The very high density of the core of earth is due to very, very high compression of the core material.